By Donald Lewis
Iowa State University Extension
Among life's interesting little critters are some that seem to be what they are not. These imposters often resemble something else, either by serendipity or by mimicry. One case of vague superficial resemblance is a tiny arachnid called the pseudoscorpion.
Pseudoscorpions are related to scorpions, and to the mites, ticks and spiders. The big difference between the groups is the small size of pseudoscorpions, usually less than 5 mm long. They are so small they usually go unnoticed. They also spend most of their lives under mulch, leaf litter, stones and tree bark and other places where they will be hard to see.
When they are noticed is when they accidentally invade homes and wander into sight. Then they attract attention because of their large pincers (called pedipalps) on the front of the body that creates the resemblance to the true scorpions. Unlike scorpions, pseudoscorpions have no sting on the end of their flat, oval abdomens.
Pseudoscorpions are not harmful. They cannot bite or sting and they do not attack the house structure, furniture or occupants. They may be an annoying nuisance, usually during the spring and summer, as an occasional “accidental invader.” Only rarely are they a chronic pest problem.
Natural habitats for pseudoscorpions include under leaf litter and mulch, in moss, under stones and beneath tree bark. They have also been reported in bird nests and between siding boards of buildings. Because they are sometimes found among books, they are also known as “book scorpions.”
Pseudoscorpions are predacious and therefore beneficial. They feed on other arthropods, particularly small insects and mites. When capturing their prey the pseudoscorpions use a poisonous gland inside the pincer-like claws to inject poison to subdue the prey. They also inject digestive fluids into the prey and later absorb the digested contents, a form of external digestion.
Special treatments for control of pseudoscorpions are usually not warranted. Only in a persistent infestation should control be attempted. It would be difficult to prevent all invasion by pseudoscorpions but sealing gaps, cracks and other points of entry may help exclude them. A light application of a household residual insecticide such as "ant and roach killer" can be made as a last resort, if warranted. In general, we have nothing to fear from these small creatures that are ecologically beneficial for our gardens and our homes.